Remote Phosphors: Philips LED bulb, Tear-down Part II
We’ve seen that Philips’ dimmable 12.5W 800 lm LED bulb emits light in the same spherical pattern we’ve come to expect from incandescent bulbs. How does it do this?
First off, let’s refer back to the unusual-for-a-white-light-bulb yellow color of the Philips bulb. Philips clearly expected some raised eyebrows from shoppers because the top text on the product package is: “WHITE LIGHT WHEN LIT,” placed right over the yellow bulb top.
[$39.97 at Home Depot.]
Your basic white high-brightness LED is based on an intensely blue LED with a dollop of phosphor directly on top of the blue emitter. The LED’s blue light strikes the phosphor which in turn emits white light, serving as a very directional point-source. This type of LED/phosphor combination is called a primary phosphor.
The yellow plastic of the Philips bulb not just a bulb-like cover over the LEDs, but the phosphor itself. Because it’s located separately from the LEDs it’s called a secondary, or remote, phosphor.
By popping off one of the three yellow plastic sections, we can see that the LEDs – which are blue, not white - are mounted vertically on the interior central column of the bulb.
By using a remote phosphor, the bulb is able to make use of the remote phosphor’s characteristic of emitting light omni-directionally and uniformly, rather than as a point source such as in a standard white LED.
Sure enough, when I power up the bulb with the yellow plastic cover removed (GAAUGH, my retinas…) we can see the intense light of the blue LEDs.
[Although I joke about the intensity of these blue LEDs, I think they are really quite nasty and shouldn’t be looked at directly without protective goggles. Kids, don’t try this at home.]
But let’s keep going and find out What Lies Beneath, although this meant destroying the bulb which caused me great mental anguish.
Here’s the top view of the bulb looking down into the wiring and connectors that take the power to the three LED boards.
I was quite impressed by the use of connectors in this bulb rather than relying on low-cost-labor for hand-soldering which has been used in the manufacturing of CFLs. For example, here’s a view of a pc board from a hand-soldered CFL:
The big solder blobs don’t inspire confidence in manufacturing quality and probably have played a role in shorter-than expected-lifetimes for some CFLs. Philips’ decision to rely on connectors makes for a more repeatable, reliable assembly process and should pay off in long-term bulb reliability.
Back to the Philips bulb: This photo shows the LED pc board and connector, as well as the thermal interface material that helps pull the heat away from the LEDs into the (fairly massive) heat sink.
Popping off the socket to expose the power management pc circuitry:
Whittling away to expose more of the power circuit. My neighbor has a machine shop, but all he needed was a hacksaw and some judicious cutting and out popped the potted circuit:
The rubbery potting compound pulled away quite easily, revealing the side of the pc board with all the magnetics and capacitors:
Drum roll, please. Here’s El Dorado – which dimming IC does it use?
The dimming LED driver IC is the 8-pin IC seen on the left. I couldn’t get a good photo of it, but the lettering looked something like CY[?]0LED which I believe is a Cypress CY8CLED 12W dimmable LED driver. Here is its Reference Design Guide. And yes, when I had it connected it to a triac dimmer, the bulb dimmed beautifully from almost zero to the full 100%.
This wraps it up for the Philips 12.5W bulb. I do have some more very interesting photos of a bulb with a related remote phosphor technology and I’ll get to that in a day or so. Just to belabor the obvious, I am quite impressed by what can be done with remote phosphors, as well as the role of connectors in high-volume electronics.